Proterozoic or Precambrian (1.5 billion to 544 million year old rocks)

Cambrian Period (544 to 505 million year old rocks)

Ordovician Period (505 to 441 million year old rocks)

Silurian/Devonian Period (441 to 362 million year old rocks)

Mississippian Period (362 to 320 million year old rocks)

Pennsylvanian Period (320 to 286 million year old rocks)

Cretaceous Period (144 to 66 million year old rocks)

Tertiary/Quaternary Period (66 million years old to present aged rocks)

Timeline Synopsis of Missouri Geology

Many books have been written about Missouri Geology in the 300 years since the first European saw the glint of galena (lead sulfide) and mistook it for silver. One website can hardly claim to outdo literally thousands of pounds of paper which add up to our sum of knowledge on the subject.

Even so, the following pages can give you an idea of what it took to make our fair state. The timeline references the generalized map by rock ages and color key. Just remember that the oldest rocks are those tending toward the red and orange on the map; the youngest ones are towards the blues and greens, and yellow (mostly river deposits) is the youngest of all. Soils of very recent origin are not shown on this map.

First, A Word about Geological Time

Geologists believe the earth condensed about 4.6 billion years ago. For the first one or two hundred million years, the earth was too hot to form rocks which survived; the oldest rock known to man in 2002 is a zircon crystal 4.4 billion years old. For these first 600 million years (known as the Hadean Eon) the earth was in a semi-molten state. The planet cooled somewhat during the Archean Eon, (3.9 to 2.5 billion years ago) and developed an atmosphere and an ocean.

The earliest life forms, bacteria and extremophiles known as Archea developed in the oxygenless atmosphere of the late Archean.

In the Proterozoic Eon (2.5 billion to 544 million years ago), the earth as we know it began to take shape. Primitive plants and inorganic processes enabled the atmosphere to reach its current level of about 21% oxygen, and familiar life forms began to develop. The earth's crust cooled sufficiently for continents to form. By the Phanerozoic Eon (beginning 544 million years ago to present) the first shelled animals capable of leaving distinctive fossils evolved. And the rest, as they say, is history. The Phanerozoic is divided into the Paleozoic, Mesozoic and Cenozoic Eras. Since more is known about Phanerozoic Eon rocks, these Eras are subdivided into the much smaller units known as Periods. Except for the Precambrian, the geological map of Missouri is based on Period names.

Assumptions to make when reading a Geological Map

The first assumption is that this map is very generalized. The smaller the mapped area, the more detailed one can be about what sort of rocks are there. Shrinking the state of Missouri to one 8.5 x11 inch page means that much detail is left out. Larger maps with greater detail, and even geological 7.5 minute quadrangle maps for some areas can be obtained for a modest fee from both the United States Geological Survey, and the Missouri Department of Natural Resources GS-RAD, the latter of whom made the map at the top of the page.

Secondly, the map at the top of this page shows only the rocks at the surface across Missouri. All things being equal, (and in the Midwest, they mostly are) the oldest rocks are laid down first, and then successive layers are added, in a generally horizontal manner, just as you would construct a deli sandwich. Just as a sandwich can be deformed by bending, tearing, shaking, or a sideways push, so rocks too, can be deformed. In addition, the top layers of a "rock sandwich" can be removed by weathering and erosion, just as a mouse might eat from the top down of your hypothetical sandwich. Geologists know what rock layers lay below the surface by examining columns of rock brought up by drilling wells, or mining.

In order to recreate the sequence of what happened to form Missouri, therefore, we have to get to the bottom of things-- and this means starting with the oldest rocks, and reading the geological record from the bottom up.

If you are a geology student, you may notice that three geological periods are missing in this lineup from oldest to youngest--with a large gap after the Pennsylvanian, we are missing the Permian, the Triassic, and the Jurassic. Rocks of these ages are nonexistent in the state as far as we know. However, it was only relatively recently that Cretaceous age rocks were correctly identified, and the first dinosaur fossils found in the state, so we can never say never, but it seems rather unlikely for other reasons that a large quantity of rocks of these three missing ages will be discovered.

For further information on those periods we do know about, please continue with the links below. I've tried to use as few technical terms as possible, but a few are inevitable. For starters, transgression means the sea covered the land; regression means the sea retreated and the land became dry again. Most unfamiliar terms will be in a good college level general dictionary. If you are stumped, have questions or comments, I can be reached at:
joschaper@socket.net